In computer operation, especially personal computer operation, it is often desirable to provide modems which allow a computer to send and receive data across conventional telephone lines. As known in the art, and to most users of computers, modems (a contraction for modulator/demodulator) are devices which convert digital signals from a computer or terminal into a modulated signal appropriate for transmission across conventional telephone lines. Most modems for personal computer use a modulation technique which is appropriate for transmission over the relatively narrow usable bandwidth of conventional phone lines.
Many personal computers in use today have a hardware architecture by which peripheral devices are connected to expansion slots on the mother board of the computer, as opposed to serial or parallel ports with plug connections which are external to the computer cover. A number of peripherals or expansion devices may be inserted into the expansion slots and the normal cover for the computer can be replaced. This arrangement allows the user to expand the capabilities of the computer without a corresponding expansion of the number of boxes and cables lying about at the computer installation site.
In the marketing of personal computers, a trade-off is often necessary between the physical size of the computer and the number of expansion slots made available to the user. Therefore, inter-slot spacing in such computers is kept to a practical minimum. Modems and other devices constructed upon a single circuit board which occupy only a single expansion slot of a small computer are desired by users.
As previously discussed, modems provide an interface between the computers a telephone network. However, telephone networks typically operate on a relatively low voltage (48 volts) compared to the operating voltage supplied to the computers (120-240 volts). Therefore it is imperative that the computer supply voltage be sufficiently isolated from the telephone network, in order to avoid damage to the telephone network. Consequently, various regulation schemes have been propagated to control the design and manufacture of modems.
One such regulation scheme is that promulgated by the British Board of Telecommunications (BABT), in the United Kingdom. This scheme requires that modems designed for connection to the phone network be enclosed by a cover having restricted vent hole sizes. Although it is probable that such a scheme was intended only for modems external to the computer, or "stand-alone" modems, nevertheless the scheme has been interpreted to apply to internal modems.
The BABT regulations require the above-referenced type of housing to enclose the telephone line interface portion of the circuit in question. However, the inventors of the present invention have found it is preferable, although not necessary, to provide an enclosure for the complete circuit board. This allows the modem designer to subsequently alter the layout of components on the computer side and telco side of the telephone line interface without requiring redesign of the bureauocratically required enclosure.
As referenced above, typical small computers with expansion slot architecture have predetermined inter-slot spacing which is as narrow as practically possible. Most typical circuit boards are of relatively small thickness with repect to the available inter-slot spacing. Therefore, users of such computers can normally extract circuit boards inserted into expansion slots by gripping portions of the relatively thin circuit board and pulling them upward. Expansion slot connectors generally require significant insertion force to insert the board, and therefore grip the inserted board rather tightly. This is done in order to insure the integrity of the electrical contacts at the circuit board/interface slot connection. However, once a significant portion of the board is encased in a plastic housing, two problems arise with respect to removal of the circuit board. The first is the fact that most plastics exhibit a rather low coefficient of friction with human fingers, and thus it is difficult to grip the plastic housing with sufficient force to pull the circuit board/enclosure assembly out of the slot. Secondly, use of a plastic enclosure over the board will normally (as it does in the case of the preferred embodiment of the present invention) cause the board/enclosure assembly to occupy almost the entire available inter-slot spacing. Therefore, even if the above-noted problem with friction could be overcome by the user, it is difficult to lower one's fingers on the surface of the enclosure sufficiently to get a good grip on the board to extract it in situations where adjacent expansion slots are also occupied.
Therefore it may be seen that a need has existed for a compact modem circuit board enclosure which satisfies the aforementioned regulations, is configured to be readily inserted and removed from a standard expansion standard slot within a computer, and provides adequate ventilation for the circuit board.